FIELD OF THE INVENTION
The invention relates to a chip module for incorporation into a basic card body of a chip card.
Chip cards are sufficiently known in the prior art. The application possibilities for chip cards are extremely varied and are still steadily increasing with the increasing computing power and storage capacity of the available integrated circuits contained therein. However, that entails an increase in the size of the integrated circuits to be incorporated (electronic components, chips).
The structure of a chip card is determined by many characteristics, some of which are prescribed by corresponding standards. That relates, for example, to the resistance of the card with respect to UV radiation and X-radiation, the mechanical load-bearing strength of the card body and of the electrical contacts, and the temperature resistance.
In most production processes for chip cards, the chip is first of all mounted onto a flexible film. The necessary card contacts are on this film. The individual chip modules are then punched out from the ready-mounted film. In that case, the chips are attached indirectly in the card through the carrier of the module. That has the advantage of keeping flexural forces away from the chip. In that case the chip is mounted with its rear side on the flexible film. The electrical terminal areas located on its front side are connected through bonding wires to bonding assemblies, which are connected through the flexible film to the card contacts. The front side of the chip, including the wires, is generally protected by a covering, for example a plastic composition, against corrosive influences. The configurations referred to above are parts of customary processes for the packing of chips in standard packages.
According to the current state of the art, chip modules for chip cards with contacts are mounted according to three basic technologies:
a) cyanoacrylate adhering technique; PA1 b) hot-melt adhering technique; and PA1 c) contact-adhesive adhering technique.
The technique referred to under c) is also referred to as pressure-sensitive adhesion. In that case, the shear stress produced by a mechanical pressure in the adhesive layer leads to a reduction in the viscosity of the adhesive which establishes or improves the physical contact with the connecting element, for example a basic card body. An example of a single-layer hot-melt adhering technique is provided by German Published, Non-Prosecuted Patent Application DE 36 39 630 A1. An example of a multi-layered cyanoacrylate adhering technique is provided by European Patent Application 0 527 438 A2.
The durability of a chip card is decisively determined in particular by the quality of the bond between the chip module and the card body. If, for example, a chip module is inserted into a recess milled into a card body and is adhesively fixed there, that may entail a significant temperature loading if, for example, a hot-melt adhesive is used.
A chip module is understood to mean a chip carrier which has eight contacts on the contact side. A semiconductor chip which is placed on the opposite side is located between those contacts of the chip card, i.e. it lies directly opposite, as seen laterally. Such chip modules are known from U.S. Pat. No. 4,625,102 and French Patent Application 2 617 668. While the cyanoacrylate adhering technique and the contact-adhesive adhering technique make do with low process temperatures for module mounting, a significant defining feature of the hot-melt adhering technique is the comparatively high incorporation temperature. Customary temperatures lie in the range from 200 to 250.degree.. A great amount of heat is introduced into the chip module in that method during the module implantation, which lasts less than 1.5 seconds. That heats up not only the adhesive layer which bonds the chip module to the basic card body, but also the chip carrier element, the semiconductor chip and the covering composition. In that case, it must be taken into consideration at the same time that the structure of a chip module includes a plurality of component parts of different materials. For example, according to U.S. Pat. No. 4,625,102 the flexible film, which is the carrier, may be formed of an epoxy resin. The contacts applied thereto on one side are formed of a metal, the semiconductor chip is formed of a crystal, for example silicon, the bonding wires are formed of metal and the covering composition provided for the front side of the chip is formed of a plastic. If that system is subjected to a major temperature fluctuation, the consequence may be so-called delaminations, i.e. separations between adjacent layers. In the case of a chip module of the type referred to, the bond, generally an adhesive bond, between chip and the carrier, is uncritical. However, brief heating up of the overall system leads to local defects in the covering composition, which are referred to as delamination, after lengthy storing and transporting times of the modules. Delaminations between the chip and the covering composition may lead to malfunctions or to the failure of the chip module. The reason therefor is that a delamination in that area causes a tearing off of electrical conductors, namely the bonding wires by which the chip is connected to the outer contacts. The above-mentioned hot-melt method of adhesively fixing a chip module into a card body entails a correspondingly high thermal loading of the chip module.
The covering compositions curing under ultraviolet light, are particularly susceptible with respect to delamination. The cause thereof are thermally induced stresses in the overall system of the chip module. Those stresses exist because of dilathermal mismatches between the semiconductor chip, the chip carrier element and the covering composition. The relevant coefficients of thermal expansion in the same temperature ranges are in the ratio 2.5:10:18. The probability of delaminations occurring increases with an increase in the size of the semiconductor chip and with an increase in the transporting and storing time. In that time, the covering composition absorbs moisture from the ambient air and that moisture leads to a material expansion, which is also referred to as swelling. The swelling of the covering composition and the brief pronounced heating up of the module when it is incorporated into the basic card body are responsible for high reject rates after incorporation. That is the case in particular with chip modules which have a chip with a large surface area. Such chips are referred to as microcontrollers and/or cryptocontrollers.
Previous measures for eliminating disadvantages from the prior art included the development, selection and trial testing of covering compositions which have a low absorption of moisture, and measures which lead to an increase in the adhesion of the covering composition, such as, for example, greater roughness of the chip carrier surface. For example, covering compositions which are provided with adhesion promoters (silane/siloxane-based) have thus been used.